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They wrote in court documents filed Monday they have found an expert witness who will testify that Autumn’s injuries were not the result of an accident and that there is evidence of “previous traumatic injury.”

Debbie Buttz, the girl’s maternal grandmother, broke into tears at the news.

“We all knew that she did it,” Buttz said Wednesday. “We don’t know what her reasoning was, but we knew she did it. All she had to do was admit it.”

Bechtel was the girl’s father’s live-in girlfriend in early 2008. On the morning of April 30, she was home alone with Autumn. Just after 8 a.m., she called 911 to say the girl had had an accident and was unresponsive.

Paramedics took the girl to Tacoma’s Mary Bridge Children’s Hospital, where she died later that day.

Bechtel later told Lakewood police detectives her 60-pound dog, Dozer, knocked Autumn off the couch and that the girl had landed on her head, court records show.

“According to the defendant, out of the corner of her eye she saw something falling back and ‘flying across the room,’ ” prosecutors wrote in court documents.

But Kiesel indicated there was a slight possibility the dog was responsible, Jerry Costello, then the county’s chief criminal deputy prosecutor, wrote to the girl’s family in explaining his decision not to pursue charges in 2008.

Costello also consulted a physician at Seattle Children’s Hospital who has studied child head injuries. He, too, said Bechtel’s explanation, while unlikely, was possible.

Prosecutors recently sent the case files to a forensic neuropathologist from Philadelphia. Dr. Lucy B. Rorke-Adams determined the girl’s death was homicide and that there is evidence “of previous traumatic injury,” court records show.

“Her report gave us the evidence we needed to file a murder charge,” Prosecutor Mark Lindquist said.

Rorke-Adams, a professor at the University of Pennsylvania School of Medicine, has published several professional reports on childhood head injuries and disease, including a 2009 paper titled, “Neck injuries in young pediatric homicide victims.”

She also has testified across the country at homicide trials involving child victims, according to news reports.